Thallus

Thallus, plant body of algae, fungi, and other lower organisms formerly assigned to the obsolete group Thallophyta. A thallus is composed of filaments or plates of cells and ranges in size from a unicellular structure to a complex treelike form. It has a simple structure that lacks specialized tissues typical of higher plants, such as a stem, leaves, and conducting tissue.

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The thallus of a lichen has one of several characteristic growth forms: crustose, foliose, or fruticose (see below Form and function of lichens). Crustose thalli, which resemble a crust closely attached to a surface, are drought-resistant and well adapted to dry climates. They prevail in deserts, Arctic and Alpine regions, and ice-free parts of Antarctica. Foliose,...

Many of the simpler fungi produce differentiated male and female organs on the same thallus but do not undergo self-fertilization because their sex organs are incompatible. Such fungi require the presence of thalli of different mating types in order for sexual fusion to take place. The simplest form of this mechanism occurs in fungi in which there are two mating types, often designated + and...

...filaments enclosed by a rigid cell wall. The filaments, called hyphae (singular hypha), branch repeatedly into a complicated, radially expanding network called the mycelium, which makes up the thallus, or undifferentiated body, of the typical fungus. The mycelium grows by utilizing nutrients from the environment and, upon reaching a certain stage of maturity, forms—either directly or...